1. Field of the Invention
The present invention is related to a multi-variable mass/flow rate transfer device (multiple variable transfer device) in which a plurality of physical signals (differential pressure signal, static pressure signal, and temperature signal) detected from a process are inputted so as to calculate mass and a flow rate by a microprocessor; the calculated mass/flow rate are outputted as, for instance, analog current signals of 4 to 20 mA to a 2-wire type transfer line, or is directed to a multi-variable mass/flow rate transfer device having communication means operated based upon a protocol as to a communication standard, which outputs digital information such as a calculation process result to a field bus.
2. Description of the Related Art
FIG. 3 is a structural diagram for indicating a conventional multi-variable mass/flow rate transfer device 10′ (refer to patent publication 1).
A differential pressure/static pressure/temperature calculating unit 11 calculates a differential pressure value, a static pressure value, and a temperature value in real time based upon signals which are detected from a process and are entered thereto. Furthermore, a differential pressure/static pressure/temperature output unit 14 is connected to an output of the differential pressure/static pressure/temperature calculating unit 11 so as to output the differential pressure value, the static pressure value, and the temperature value, which are transferred to an external unit (not shown), respectively.
Further, an instantaneous flow rate calculating unit 12 is connected to an output of the differential pressure/static pressure/temperature calculating unit 11 so as to calculate an instantaneous flow rate in real time from the differential pressure value, the static pressure value, and the temperature value. Furthermore, an instantaneous flow rate output unit 13 is connected to the output of the instantaneous flow rate calculating unit 12 so as to output the value of the calculated instantaneous flow rate, which is transferred to the external unit.
In addition, an instantaneous flow rate alarm unit 17 is connected to the output of the instantaneous flow rate calculating unit 12, and when an instantaneous flow rate value becomes a predetermined value, the instantaneous flow rate alarm unit 17 produces an alarm, and this alarm is transferred to the external unit. Further, a differential pressure/static pressure/temperature alarm unit 18 is connected to the differential pressure/static pressure/temperature calculating unit 11. When a differential pressure value becomes a predetermined value, the differential pressure/static pressure/temperature alarm unit 18 produces an alarm; when a static pressure value becomes a predetermined value, the differential pressure/static pressure/temperature alarm unit 18 produces an alarm; and when a temperature value becomes a predetermined value, the differential pressure/static pressure/temperature alarm unit 18 produces an alarm, and then, these alarms are transferred to the external unit.
FIG. 4 is a characteristic diagram for representing a threshold value of an instantaneous flow rate alarm unit 17 of the conventional multi-variable mass/flow rate transfer device 10′ of FIG. 3. An abscissa of the characteristic diagram indicates time, whereas an ordinate thereof shows an accumulated value of instantaneous flow rates which are produced by the instantaneous flow rate calculating unit 12.
In the drawing, while a characteristic “F1” corresponds to a characteristic in the case that an instantaneous flow rate “A” is substantially constant, both an elapsed time “t” and an accumulated flow rate “S” are directly proportional to each other, and satisfy the below-mentioned formula (1).S=A·t  (1)
Further, another characteristic “F2” is such a characteristic that when an instantaneous flow rate “A” is substantially constant, and is equal to an upper limit “+a” of a flow rate precision percentage ±a [%], both an elapsed time “t” and an accumulated flow rate “S” become a direct proportional relationship, an inclination is increased, and satisfy the below-mentioned formula (2).S=A·t·((100+a)/100)  (2)
Further, a characteristic “F3” is such a characteristic that when an instantaneous flow rate “A” is substantially constant, and is equal to a lower limit “−a” of the flow rate precision percentage ±a [%], both an elapsed time “t” and an accumulated flow rate “S” become a direct proportional relationship, an inclination is decreased, and satisfy as below-mentioned formula (3).S=A·t·((100−a)/100)   (3)
Then, when the instantaneous flow rate “A” becomes larger than the upper limit “+a”, the instantaneous flow rate alarm unit 17 produces an alarm, whereas when the instantaneous flow rate “A” becomes smaller than the lower limit “−a”, the instantaneous flow rate alarm unit 17 produces an alarm.
Further, when the instantaneous flow rate “A” is substantially constant and is equal to the flow rate precision percentage ±a [%], at a time instant “T1”, there are some possibilities that the instantaneous flow rate “A” is varied from an accumulated flow rate “S3” (=A·t·((100−T1)/100) up to another accumulated flow rate “S2” (=A·t·((100+T1)/100).    [Patent publication 1] JP-A-2005-190461    [Patent publication 2] JP-A-2003-57098
However, in the conventional technical idea of FIG. 3 and FIG. 4, there is a problem such that when the elapsed time “t” is increased, a difference (A·t·((100+2·a)/100) between the accumulated flow rate S2 and the accumulated flow rate S3 is increased. Concretely speaking, if the elapsed time “t” is increased two times; then the difference between the accumulated flow rate S2 and the accumulated flow rate S3 is also increased twice.
As a consequence, the following problem occurs: That is, it is not possible to set in high precision as to whether or not an accumulated flow rate is appropriate based upon the threshold value of the instantaneous flow rate alarm unit 17.